Preparation of fine cellulose flour



1944' E. H BALZ ET AL 2,362,528

PREPARATION OF FINE CELLULOSE FLOUR Filed June 3. 1942 Em/7 A. 50/2ATTORNEYS Andrew W Aassay NTOR:

Patented Nov. 14,1944

, zssaszs raaraaa'rron or me caLL Losn nova Emil a. mu, Andrew w.Kailay, and wmismn. Williams, Toledo, Ohio, aeeignors. by memeassignments to Hblny-(lwens-Ford pony, a corporation of Ohio Glass Com.-

Application 3, i942, Serialhlo. 445,656 I 2 Claims.

The invention relates to the preparation of fine cellulose flour that isuseful as a filler for synthetic resin compositions, such as moldingcompounds and adhesives, and other products, such as insecticides, forwhich fine cellulose fiour is now used.

Wood and other forms of cellulose are difilcult to reduce to a finepowder because of the fibrous nature of cellulose. Cellulose is a toughand flexible material, and the individual cellulose fibers are believedto have a higher tensile strength than steel. For many applicationsextremely fine cellulose fiour is desirable, and such cellulose flour isdiflicult to produce by grinding because of the toughness of thecellulose fibers. H

Cellulose can be reduced to a fine flour by prowhich the cellulose isground by the impact of tumbling members, such as a ball mill or a tubemill. cylinder containing steel or porcelain balls which tumble as thecylinder rotates and exert a grinding action upon material contained inthe cylin- A ball mill consists essentially of a rotary longed grindingin a tumbling impact mill in put of cellulose flour. I advantages areapparent from the description in Y which reference is had totheaccompanying drawing. I

Figure I of the drawing illustrates a preferred form of apparatus forcarrying out the invention.

Figure 11 is a vertical section taken on the line II-II of FigureI.

This specific drawing and the specific description that. follows merelydisclose and illustrate, the invention, and are not intended to imposelimitations upon the claims.

After cellulose has been ground in a ball mill or other tumbling impactmill for several hours. only a small proportion of particles finer thanthe meshes in a 2'10 mesh screen can be recovered, and it is necessaryto prolong the grinding in the tumbling impact mill for an unreasonablelength of time in order to reduce the greater proportion of thecellulose to a fiour of such fineness.

In accordance with the invention, it has been discovered that finecellulose particles cannot beder. In a continuous process a grindingoperation of this character is ordinarily performed by a tube mill.Tumbling members in the form of tubesor rods may be employed in place ofballs in such a mill. A continuous mill of this type is an elongatedcylinder rotating on its longitudinal axis, which is inclined slightlyso that when material is fed at thehigher end, the ground product willflow at the lower end.

satisfactorily separated from coarser cellulose particles by screening.Cellulose particles which are fine enough to pass through the meshes ofa. screen, because of their fibrous nature, interlock together andbridge over the meshes of the screen. Dry screening thusprovides anunsatisfactory separation of the desired fine particles. l Eifectiveseparation by screenlng'can be accomplished if the ground cellulose ismade up in a Such a tumbling impact mill must be of relatively largesize in order to provides. reasonable out-put because the materialpasses through they mill quite slowly. Thus a tumbling impact mill is avery expensive piece of production equipment.

However, a tumbling impact mill .is the only type oflf mill in whichcellulose can be groundto a fine our.

A common form of commercial mill, known as an attrition mill, isordinarily employed for the operation of reducing a fine saw-dust to theform of coarse wood flour. The finest wood flour that slurry with waterand screened wet. Such satisfactory screening is not an economicaloperation, however, because of the expense of making up the slurry andsubsequently drying the product.

In accordance with the invention, therefore,

' the desired fine particles are'removed from the ground mass in eachstep by air separation. The invention is based upon the-discovery thateven air separation does'not provide satisfactoryremoval of the fineparticles. It has been discovered that because of static charges on thefine I particles or because of enmeshing fibrous ten-' tacles on theparticles, there is a tendency for can be produced by means ofanattrition mill 45 groups of fine particles to cling together in is socoarse that less than 5 per cent of it will clumps. Such clumps do notgo of! with the fine pass through a 100 mesh screen. The fine woodparticles when air separation is carried out, but fiour that is desiredfor most applications of the are removed with the coarse particles ortailings. type mentioned above is wood flour so fine that It has beendiscovered that if the tailings from. substantially all of it will passthrough-a 270 mesh 50 such an air separation are subjected to a simplescreen. grinding operation, the clumps of fine particles The principalobject of the invention is to probecome separated so that a large yieldof addivide a method of producing cellulose flour by tional fines can beobtained by a further air sepwhich a tumbling impact mill of relativelysmall a aration. Thus when a yield of 20 per cent fines size can be usedto provide a relatively large outis obtained by air separation ofcellulose that has More specific objects and I -is a simple type ofrotor impact mill, and the ,tailings irom the second air separationarereturned to the tumbling impact mill.

The principalapplication oi the invention is in the production of finewood flour. In the production of wood flour by the present method, finesawdust, obtained by passing chips, shavings and coarse sawdust througha rotary cutter, is ground in an attrition mill to produce coarse woodflour which is in turn led to the tumbling impact mill used in carryingout the present invention.

The present method can b used for comminuting any form of cellulose,such aswalnut shelhfiax shives, a bleached sulflte pulp, such as alphacellulose, or any other cellulosic material.

When the method is carried out by means of the apparatus illustrated inthe drawing, wood waste, usually in the form oi. a mixture of chips,shavings and coarse saw-dust, is passed through a rotary cutter-l whichconvertsit to fine sawdust. The fine saw-dust is suitable for processingin an attrition mill 2 to which it is conducted by a chute I. Theattrition mill reduces the fine saw-dust to a coarse wood ilou'r, Theattrition mill is used to eflect a preliminary grinding oi the materialin order to make possible a more rapid out-put from the tube mill 4 towhich the coarse wood flour'is fed from th attrition mill through aninclined chute 5. Although the attrition' mill is capable of carryingout'only a pre-j liminary and relatively coarse grinding oi thematerial, the out-put ofa small attrition mill in pounds per houris'sreater than the out-put of a large tubemill costing 20 to 30 times asmuch as the attrition mill.

The tube mm 4, should be provided with a jacket of the usual type. forcooling water.

The ground product is discharged from the tube mill 4 into a storagehopper provided with a gate 1. From time to time. the gets I is openedslishtly to cause the contents oi the hopper O to be emptied slowly intoa chute from which the product is conveyed pneumatically by a blower Ithrough a conduit I. to an air separator II. The surplus air from theair separator ll returns through ducts I 2 and II to the blower.9. A bag5 Ill is used to collect the fines, which are the finished product.After each emptying of the storage hopper 6, the gate 1 is closed,a'gate ii in a tailings hopper I8 is opened and a divertin vane 11 isturned to the left in order to cause the trailings from the hopper It tobe fed to a small imp mill it, having an impact rotor l8. By

means of a blower I! the product from the imp mill I8 is conveyedpneumatically through a vertical conduit to the air separator H, and

the surplus air from the air separator is returned to the blower l9through the return air ducts I2 and ii. A gate 22 in the discharge chute23 for the fines is provided in order to permit the discharge of thefines to be shut ofi momentarily while an empty bag is substituted ior a'full bag.

The re-grinding of the tailings in the imp mill l8 and the airseparation of the resulting product are continued, the taillngs from thesecond air separation being returned to the imp mill, until thedischarge of fines from the air separator has substantially ceased. Thenthe diverting vane II is turned to the right and the grinding of thetalllngs in the imp mill and air separation of the resulting product arecontinued until all of the so tailliigs have been fed back to the tubemill 4. The gate ll of the tailings hopper I8 is then closed, and thesystem is ready for another emptying oi the storage hopper.

Various embodiments may be devised to meet various requirements.

Having described the invention, we claim: a a p 1. A method oi producingcellulose flour that comprises grinding cellulose in a tumbling impactmill, air-separating the fines from the resulting- Product, resrlndingthe tailings from the air separation in a rotor impact mill, andair-separatin in: to said tumbling impact mill the tailings, from thesecond air separation.

msmneanz.

ANDREW w. KABSAY. wnmmu p. WILLIAMS.

